Recently, there have been an increased number of vehicles equipped with a yaw rate sensor for measuring a vehicle's angular velocity around its vertical axis. The yaw rate sensor includes an oscillating body periodically oscillating in a first direction and measures the angular velocity by detecting an oscillation in a second direction perpendicular to the first direction. The yaw rate sensor outputs a sensor signal having a voltage corresponding to the detected angular velocity. Generally, the sensor signal is synchronously detected based on an oscillation frequency of the oscillating body. The detected sensor signal is full-wave-rectified and then filtered so that necessary signal components can be extracted. For example, JP-A-2003-214893 discloses a filter circuit for filtering a sensor signal outputted from a yaw rate sensor.
The signal processing task described above is performed by using a processor such as a digital signal processor (DSP) or a central processing unit (CPU). Typically, the processor performs the signal processing task and another task such as a communication task in parallel. When the communication task needs to be performed, the processor receives an interrupt request. Upon reception of the interrupt request, the processor suspends the signal processing task and then starts to perform the communication task as shown in (a) of FIG. 11. As a result, the signal processing task is kept in a wafting state indicated by diagonal lines in (a) of FIG. 11, until the processor completes the communication task.
An edge of a squire drive signal having a frequency equal to the oscillation frequency of the oscillating body needs to be detected in order to perform a synchronous detection of the sensor signal. However, as shown in (b) of FIG. 11, the edge detection may result in failure depending on the timing of the interrupt request. If the edge detection results in failure, the detected sensor signal is incompletely rectified. As a result, the detected sensor signal may contain different phase components as shown in (c) of FIG. 11.
For example, the edge detection failure may be avoided by causing the processor to operate at high frequency or by using a coprocessor. However, this method increases consumption current, circuit size, and cost.